In the photographic industry, the oxidation of metallic silver in photographic images or negatives to silver ion is known as bleaching. A desirable bleaching agent will react rapidly with silver and then react rapidly with air to regenerate the bleaching agent. Ferric compounds have been used for this purpose for decades. The ferric compound in the most widespread use today is ferric ammonium ethylenediaminetetraacetate because of its desirable redox properties and ease of preparation from inexpensive commercial chemicals (ferrosoferric oxide; ethylenediaminetetraacetic acid (EDTA); and ammonia).
As described by Donovan and Surash (U.S. Pat. No. 3,767,698, published Oct. 23, 1973) and by Svatek, et al. (U.S. Pat. No. 4,364,871, published Dec. 21, 1982, and U.S. Pat. No. 4,438,040, published Mar. 20, 1984), a ferric ammonium EDTA solution can be prepared by reacting iron oxide with a stoichiometric excess of ammoniated EDTA in an aqueous mixture at temperatures below 100.degree. C. for less than three hours, followed by pH adjustment, aeration, and filtration. A more recent variation of the method, described by Thunberg, et al. (U.S. Pat. No. 5,110,965, published May 5, 1992), describes the use of ferrous salts to catalyze the reaction between the iron oxide and ammoniated EDTA.
Whereas chelate solutions made by the aforementioned process are useful for photographic bleaching, over time they show a tendency to precipitate fine, black, particulate matter which might be detrimental to the quality of photographs prepared using said solutions and which, in any case, may reduce customer acceptability of the product. It would be desirable, therefore, to produce ferric EDTA solutions, as well as other ferric-aminopolycarboxylic acid chelates, which are more stable against the formation of such dark particulates.